Bloodshed frequently erupted between them. According to Shiite belief, the Caliphs were trying to wipe out Ali's line of succession by killing off his heirs, the imams, one by one over the first few centuries of the faith.

Pious Shiites believe that the 10th and 11th of Ali's heirs, Ali Hadi and Hassan Askari, are buried in the shrine in Samarra. Shiites teach that both were poisoned by agents of the Caliph in the 9th century in an attempt to wipe out Ali's line once and for all.

But Shiites believe Askari had a young child, Mohammed al-Mahdi, who disappeared into a cellar near the site of the shrine before he could be killed. Shiites believe Mahdi, the 12th or "hidden imam," will reappear, and that his return will herald a new age, avenging the wrongs done to his ancestors and to the Shiite faithful who have suffered under the rule of Sunni governments for centuries.

"Mahdi is expected to return one day as a messiah," Nakash said. The shrine complex is considered a tangible link to Mahdi.